Recent innovations in Magnetic Resonance Imaging (MRI) technology has allowed IMRIS to develop new inter-operative imaging techniques which allow a patient to be imaged while undergoing surgery in an operating room. This is done by bringing an MRI system into the operating room along ceiling mounted rails at a rate of up to 6.4 cm/second. The technology has the advantage of giving surgeons immediate feedback for an ongoing operation which they did not previously have access to. However, this beneficial new technology creates potential hazards which do not exist in traditional MRI systems. These hazards are due to the fact that MRI employs extremely strong magnetic fields of up to 3 tesla in a clinical setting; more than enough to turn a ferromagnetic object into a dangerous projectile.
MRI is typically performed in a designated diagnosis room with a strict procedure in place to prevent any ferromagnetic object from entering the room. Surgery being performed in an operating room often requires ferromagnetic objects which may become hazardous once an MRI system enters the room. The current method of preventing an accident uses a checklist containing all items which need to be brought outside of either the 5 or 50 gauss line, depending on the object. A gauss line is a region painted on the floor of an operating room indicating safe zones where the magnetic field will be below 5 or 50 gauss. This current system is vulnerable to human error and does not monitor the source of the hazard, the presence of a strong magnetic field.
Magnetic Resonance Imaging or MRI is a form of medical imaging involving the use of an extremely powerful magnetic field. The magnetic flux density can be up to 3 tesla or 30 000 gauss in a clinical setting. The tesla is the SI unit of magnetic flux density or magnetic field. The gauss is a non-SI unit of magnetic field and is more commonly used to describe the strength of a magnetic field by those working with MRIs; one tesla is equal to ten thousand gauss. The theory of how the actual MRI works is not the focus of this report although a brief summary follows. MRI creates a strong magnetic field within the imaging suite using a large superconducting magnetic. It should also be understood that the magnet does not have the capability of being turned off and on easily and therefore the magnet is assumed to be always on. Although the physical size of the magnet varies by model, a common outer diameter is approximately 1.9 meters with a bore of 0.7 m in diameter. When a patient is placed in the bore or center of this magnet, the magnetic field will align the spin of the body's protons in either a parallel or anti-parallel direction with reference to the magnetic field. An RF pulse at 63.65 MHz for a 1.5 T system or 123.2 MHz for a 3 T system is then sent into the body to flip the spin of the protons off of the magnetic field axis and into another axis dependent on the power of the RF pulse. The protons which are flipped are now forced back into alignment with the magnetic field in a rotating motion similar to a spinning top. The process of the protons spin being realigned with the magnetic field creates a small decaying RF pulse which is picked up by a receive coil. The coil then sends the data back to a computer which does a 2-dimensional Fourier transform, creating the magnetic resonance image.
High field intra-operative MRI is becoming established in neurosurgery to capture surgical target displacement as a result of brain-shift and for post-operative residual tumor identification. One suite design employs a movable MRI scanner that can enter the operating room (OR) on overhead rails. This level of integration has the benefit of eliminating patient movement between pre-operative and post-operative imaging but entails other operational constraints. For example, preparatory pre-imaging steps are added to the clinical workflow, including draping the patient to maintain sterility at the surgical site. Surgical staff use paper and/or computer-based checklists to ensure that the MR imaging environment is RF-quiet and that appropriate safety pre-cautions are taken. Safety-related activities include moving MR-conditional and MR-unsafe, e.g. ferromagnetic, equipment to the exclusion zone bounded by the 5 gauss field line. Equipment moved may include boom-mounted surgical lights and monitors, anaesthesia machines, patient monitors, carts and navigation systems.
Monitoring equipment and tools within an MRI-integrated OR relies largely on proper training and minute-to-minute vigilance of the OR staff. It is clearly advantageous to mount magnetic field sensors on equipment to create a secondary and automatic means to monitor safety hazards in this environment. However, existing audible magnetic field sensors are fairly large in size and cannot be easily mounted to third party equipment. Furthermore, these sensors work individually using local audio alarm, are designed for larger structures and provide a limited number of alarm thresholds. In summary, to date there has been a lack of integration between computer-based checklists, room control systems and miniaturized distributed magnetic field sensors.
When a movable MRI scanner is used intra-operatively, there are safety concerns and impacts to the existing workflow. That is the scanner introduces a high magnetic field to the operating room. Materials that are attracted to magnets (ferromagnetic) may become projectiles causing injury or death to the patient, to hospital staff or to the surgeons.
As a result of the safety concerns, hospitals have introduced protocols that must be followed before the MRI scanner enters the room. The protocols include counting and moving ferromagnetic instruments from the center of the OR to a zone that experiences less than 5G of magnetic field, moving light and camera-booms to the 5G safety zone and moving other objects outside of the room or to the 5G safety zone.
In some instances objects may be tethered before the scanner enters the room. When imaging is completed, the instruments, booms and other objects are moved back into their position for surgical use.
The OR can be a high stress environment so that, despite training and practice, some tasks required for safety may be omitted at some times.
In practice, commercial MRI safety systems exist, for example from MedNovus and Kopp Development, which are generally large, fixed-position ferromagnetic detectors that act as a portal into the room. Persons with ferromagnetic materials on or in their person act to trigger an alarm as they enter. This is the current state of the art.
For example, in U.S. Pat. No. 7,489,128 (Kopp) issued Feb. 10, 2009 is disclosed a protection arrangement for association with an operable MRI apparatus located within a room that has an access opening, the MRI apparatus providing a residual magnetic field that extends to a location of the opening, the arrangement including: a detector for passively monitoring the residual magnetic field at the location of the opening, the detector includes an array of passive magnetic field sensors arranged about the periphery of the opening in a spaced arrangement such that each sensor is associated with a different portion of the access opening, the field changing in response to a presence of ferrous material at the opening, each of the sensors including means for outputting a signal indicative of the ferrous material responsive change in the magnetic field at the associated portion of the access opening; and means for receiving the change indicative signals, for determining whether the change indicated by at least one of the change indicative signals exceeds a limit and for providing a safety response that addresses the condition of ferrous material at the opening of the room within which the MRI apparatus is located upon determination of at least one threshold being exceeded.
A system similar to the MedNovus and Kopp systems has been developed by David Hoult of National Research Council Canada and is disclosed in U.S. Pat. No. 7,414,400 issued Aug. 19, 2008.
US Published Application 2007/0132581 (Molyneaux) published Jun. 14, 2007 discloses a system for ferrous object and/or magnetic field detection which detects a given magnetic field strength around a MRI machine and alert users to the field's presence. The magnetic field warning system can rely on a single badge that warns its user. The badge utilizes an RFID system which can turn the badge on when it enters the MRI room and off when it leaves the MRI room. The badge is worn by a person, located on or near a ferrous object, embedded in clothing, or located in other positions convenient to a user. The detector, the power supply and the user interface are utilized in a single package providing a badge type concept. This is now issued as U.S. Pat. No. 7,696,751 issued Apr. 13, 2010.
U.S. Pat. No. 7,113,092 (Keene) issued Sep. 26, 2006 discloses an apparatus for detecting ferromagnetic objects in the vicinity of a magnetic resonance imaging scanner. The apparatus comprises primary sensors adapted to measure a magnetic field, arranged in communication with a signal processor configured to identify temporal variations in the measured magnetic field due to the movement of a ferromagnetic object within an ambient magnetic field and to provide an output indicative of the presence of a ferromagnetic object in the vicinity of the primary sensor. The apparatus further comprises secondary, non-magnetic, sensors adapted to detect the movement of objects in the vicinity of the primary sensors in order to reduce false alarms. The output from the signal processor may be used to operate an audible alarm, a visual alarm, an automatic door lock or a physical barrier.